Electronic message systems that transmit so called electronic mails (e-mails) have become very important as a quick and efficient means of exchanging information. In such systems, one user can compose a message and send it to another user by specifying that user's address. This message is then translated into a format that is suitable for the transmission system to which both users are connected, and then routed through said transmission system to the addressee.
A well known system that supports such an electronic message system is the so called internet. Besides this global system, there also exist networks which are restricted to a given number of subscribers at a given location, e.g. a given number of users in one company, where such systems are referred to as intranets. It is also possible to combine local and global message transfer systems, e.g. by letting the user in an intranet send messages to other users in the intranet, or send out messages to subscribers of the global net through a so called gateway, i.e. a connection point between the intranet and the global network.
Such networks therefore allow communication between a message source and one or more message receivers. With respect to the communication between the source and the receiver, two questions need to be answered, namely what can be transferred and how can it be transferred. The first question refers to the syntax and the second to the protocol. In the presently used network communications systems described above, two major protocol standards have emerged, namely the so called Simple Mail Transfer Protocol (RFC 821) and the OSI Message Handling System (X.400 series). In order to perform the transferring of data, both of these protocols assign so called user agents as communication front ends for any given user. FIG. 6 schematically shows an example of a message handling system based on these protocols. A user agent will collect outgoing messages and incoming messages. In order to send a message, the user agent communicates with a transfer agent, which in turn handles the routing of the message through the network to the receiver's user agent. On the receiving side, the message is then routed to a so called mailbox by a transfer agent. The messages collected in the mailbox can then be called up by the receiver, via his user agent. The source or sending unit A, the network B and the receiving unit C form a communication system. As indicated by the identical structure shown in FIG. 6, both units A and C can send and receive messages.
The syntax of the transmission units or messages is generally divided into two parts, namely a header and a body. While the body carries the actual contents of the message, i.e. the information that the source desires to communicate to the receiver, the header contains the information relating to the message itself, e.g. the author's and receiver's address, as well as information concerning the time or date and the subject of the message. The body can consist of plain text or of binary encoded information representing any kind of computer data format. The transmission of several body parts that all belong to single message is also supported. A message that is divided in this way, is then reassembled at the receiver's end. For the two protocol standards mentioned above, the syntax is specified in RFC 822 and RFC 1521/RFC 1522 for RFC 821 and X.419 for OSI. This is disclosed in detail in the following documents, so that no description is given in the framework of this application.
Crocer, David H., RFC 822: Standard for the format of ARPA Internet text messages, DDN Networking Information Center, SRI International, August 1982. Borenstein N., Freed N.,RFC 1521: MIME(Multipurpose Internet Mail Extension)Part one: Mechanisms for specifying and describing the format of Internet message bodies, DDN Network Information Center, SRI International, September 1993. Moore K., RFC 1522: MIME(Multipurpose Internet Mail Extensions)Part two: Message header extensions for non-ASCII text, DDN Network Information Center, SRI International, September 1993. CCITT Study Group VII, Data Communication Networks: Message Handling Systems(MHS), volume VIII, Recommendations X.400-X.420 of the Red Book Series, International Telecommunication Union, 1984. CCITT Study Group VII, Data Communication Networks: Message Handling Systems (MHS), volume VIII, Recommendations X.400-X.420 of the Blue Book Series, International Telecommunication Union, 1989.